Filtering out the noise surrounding Stage 4

At its triennial meeting in Montreal in early October, the ICAO Assembly–which includes representatives from all 187 ICAO member nations–approved a more flexible approach to the application of aircraft noise regulations.

Currently there are three noise standards being observed internationally. Called stages in the U.S. and chapters by ICAO, but otherwise identical, the present standards are numbered 2, 3 and 4, with each being more demanding than its predecessor.

Generally, aircraft below 75,000 lb mtow are exempt from any non-compliance phaseouts, so that corporate aircraft, until the relatively recent introduction of the GV, Global Express, BBJ and ACJ, have not been affected. These four larger airplanes do, however, fully meet Stage 4 requirements. “Most business aircraft are very quiet,” said Bob Shuter, director of international aviation for Transport Canada. “With few exceptions, they meet the latest noise standards.”

Today’s standards are calculated in accordance with a complex formula that takes account of each individual aircraft’s mtow at the time of its first certification, its number of engines and several other factors. These calculations are then verified against measurements by ground monitoring equipment during flight test.

The measurements are expressed in terms of effective perceived noise decibels (EPNdB) taken at three separate locations–6,000 ft from the threshold below the approach path, 1,500 ft offset laterally from the runway midpoint and at a flyover point 20,000 ft from the start of the takeoff run. The three EPNdB values are then combined to produce a cumulative value for the specific aircraft type and model.

In the world of EPNdBs, the bigger the number, the louder the noise and the lower the stage rating. As an extremely general rule of thumb, airplanes creating around 115 EPNdB sideline, or lateral, noise preceded the introduction of Stage 2. (There never was a formal Stage 1, since the Stage 2 civil criteria were specifically developed to be quieter than post-WWII military turbojets and the earliest Boeing 707s and DC-8s.) Those producing about 105 EPNdB met the Stage 2 standards, while those that subsequently got down to around 95 EPNdB qualified as Stage 3 compliant.

Stage 4, which was approved earlier this year, is only marginally quieter than Stage 3, according to Shuter, and includes almost all Stage 3 aircraft except hush-kitted aircraft. And there are also oddities: on rare occasions, a lightly loaded Stage 2 airplane can actually be quieter than a Stage 3 airplane at maximum takeoff weight, but remember that stage classes are calculated on an aircraft’s mtow at its original certification, regardless of its service weight on a given day.

And what’s a decibel? It’s a measure of noise intensity, where a three-decibel increase produces a doubling in intensity, and vice versa, on the test instrumentation. But to the human ear, a three-decibel change either way isn’t really noticeable, and it takes about a 10-dB increase for most of us to perceive what seems to be a doubling in the noise level, and vice versa. This in turn gave rise to the 10-dB spacing between the current stages.

The Noise RulesThe ICAO noise rules include wide compliance overlaps, which means that in many parts of the world today, Stage 2, 3 and 4 aircraft can be found flying in the same airspace and operating out of the same airports. But not in every part of the world. While current Stage 2 aircraft were to be phased out of worldwide service by April 1 next year, after which all aircraft would have to meet either Stage 3 or 4 standards, Europe and the U.S. have already banned Stage 2 airplanes, with the U.S. ban taking effect, with some exceptions, from Dec. 31, 1999.

Last year, however, the 15 nations of the European Union took matters one step further and proposed extending their ban to prohibit the use of previous Stage 2 aircraft that had been hush-kitted and recertified to Stage 3 standards, such as hush-kitted 727s, early 737s and DC-9s, when such aircraft had not been previously individually registered in the EU. (This is sometimes called the nonaddition rule, which was a major factor in the recent Van Nuys, Calif. case.) Subsequently, some European airports suggested that a similar nonaddition ban should be extended to Stage 3 aircraft, well in advance of the scheduled end of Stage 3 operations in 2006.

At the same time, many aircraft operators in less developed parts of the world were beginning to question whether stringent noise regulations, primarily written in response to the concerns of residents of densely packed communities around airports in industrialized countries, were even necessary in their more sparsely settled home environments. National representatives from Africa, South America and several member states of the former Soviet Union pointed out that many of their operators could not afford the cost of buying newer, Stage 3- or Stage 4- compliant aircraft simply to meet a demand that arose only rarely in their operational areas.

These views, aside from the transatlantic war of words that followed Europe’s hush kit and nonaddition proposals, accelerated the efforts of ICAO’s Committee on Aircraft Environmental Protection (CAEP) to find a compromise solution that would encourage reductions in noise and emissions while recognizing the different operating environments in many parts of the world. The CAEP, made up of specialists from a cross section of nations, subsequently developed a new concept called the “balanced approach” to the problem.

Unlike the previous rules, which have been based essentially on just engine noise, the balanced approach calls on airport authorities to weigh and document four separate elements and also consult stakeholders before deciding on an aircraft’s admissibility.

These elements are reductions of noise from the engines, as well as land-use planning, noise-abatement operational procedures and aircraft operating restrictions.

However, CAEP’s recommendations, which were adopted by the assembly, stressed that operating restrictions, such as noise curfews for certain aircraft types, should be used only as a last resort, after all other elements had been applied. Benefit/cost analyses by CAEP also concluded that Stage 2–or Stage 3–aircraft should not arbitrarily be phased out of either worldwide or regional service, except under specific bans by individual states or airports.

Is today’s Stage 4 the end of the line in noise reduction? Almost certainly not. At the ICAO meeting, airport and environmental representatives suggested that it was not too early to start planning for an even more rigorous standard to replace Stage 4. Interestingly, such a standard would probably result in the aerodynamic noise from the gear, flaps and other turbulence-creating items completely masking the engine noise while on approach, although the engines would certainly still predominate during takeoff and initial climb.